The present invention relates to methods for preparation of ceramic products. In particular, the present invention relates to methods wherein compositions comprising ceramic powders are formed into a desired shape and fired to prepare the final densified product.
Current processing methods generally employ ceramics particulates dispersed in a suitable medium with volume fractions of solids close to or below the powder's maximum packing fraction. Techniques such as injection molding have found application for heat engine components (turbine rotors, stators, vanes, transition ducts, back-shrouds, etc). Other common techniques, such as tape casting and plastic extrusion, are also being used to prepare products such as electronic packaging.
Despite their widespread applications, these known techniques have significant limitations. In particular, with the exception of injection molding, the known techniques are generally not adequate to permit the preparation of ceramic products having complex shapes.
Most advanced ceramics are formed as powder compacts that are made dense by a heat treatment. Injection molding suffers from the large (between 35 and 50 volume-%) polymer content that must be slowly removed prior to high-temperature processing. Typical industrial practices for making complex-shaped ceramics have used organic liquids for dispersing fine ceramic particulates with polymeric binders and plasticizers (which may comprise as much as 50 vol. %) for easy forming and handling. Environmental restrictions and economic concerns strongly encourage the development of alternative processing methods which do not employ organic solvents or polymeric additives. In addition to their potential for toxicity, the polymeric binders and plasticizers used in injection molding pose several processing problems with respect to incomplete binder burn out (resulting in residual impurities and defects) and excessive burn out time (about 40 to 50 hrs/cc). During the removal of additives, the ceramics may undergo substantial shrinkage and distort from their desired shape. In addition, impurities left behind after binder burnout (which is often incomplete) may severely limit the mechanical properties of the ceramic. Accordingly, there is a need in the art for methods to prepare densely-packed compositions consisting essentially only of ceramic powders so as to obviate the serious problems associated with binders and plasticizers.
Powder processing involves four basic steps: (1) powder manufacture; (2) powder preparation for consolidation; (3) consolidation to an engineering shape; and (4) densification and microstructural development to eliminate void space and produce the microstructure that optimizes properties. Each step has the potential for introducing detrimental heterogeneities that either persist during further processing or develop into new heterogeneities during densification and microstructural development. Many microstructure heterogeneities stem from the powder itself; agglomerates are a major heterogeneity in powders, as are inorganic and organic inclusions. Heterogeneities are responsible for both dielectric and mechanical breakdown; with respect to the latter, each heterogeneity is a stress concentrator, and thus a potential flaw which can initiate failure prematurely. Current processing methods inherently lack a clear approach for controlling microstructure heterogeneities and uncontrolled phase distributions. Therefore, there is also a need in the art for methods which optimize processing reliability by minimizing heterogeneities and uncontrolled phase distributions.
In the search for approaches to improve known methods for preparation of ceramic products, significant consideration has been given to the nature of the interaction between ceramic powder particles suspended in a liquid medium. This interaction has generally been considered to consist primarily of electrostatic interparticle repulsive forces and van der Waals attractive forces [see generally, R. O. James, "Characterization of Colloids in Aqueous Systems," Advances in Ceramics, Vol. 21, pp. 349-410 (1987)]. Van der Waals interparticle potentials are always attractive between like particles. In the absence of any repulsive potential, the van der Waals potential produces very strong, attractive interparticle forces at small (&lt;5nm) interparticle separations. In this situation, particles which are initially separated and free to move are attracted to one another and quickly attach to first form small, low density clusters. This cohesive, touching network (a flocced network as schematically illustrated in FIG. 1a) requires great effort to break apart. Moreover, particles colliding with nascent agglomerates as are formed in such a flocced network are unlikely to become associated with the agglomerates at precisely the position required for formation of a close-packed configuration. Therefore, the attractive forces which created flocculation tend to prevent the achievement of the densest possible packing of powder particles.
To avoid flocculation, repulsive interparticle potentials sufficient to overcome the attractive van der Waals potential must be introduced. Long range, repulsive, electrostatic interparticle potentials are developed when a surface becomes charged. Charged oxide surfaces can be produced in water when the surface reacts with either H.sub.3 O.sup.+ or OH.sup.- ions. By controlling the pH, a net surface charge is developed which is positive (acidic conditions), neutral, or negative (basic conditions). The pH producing the maximum surface charge (either positive or negative) depends on the surface chemistry and its equilibrium with H.sub.3 O.sup.+ or OH.sup.-. The surface is neutral at an intermediate pH, where the surface contains equal proportions of positive and negative sites, as well as neutral sites. Some of the ions in the solution with an opposite charge relative to the surface (known as counterions) are attracted by the surface to form a diffuse layer. Counterions do not chemically bond to the surface, but hover in solution near the surface in an attempt to shield the surface charge. For a given surface chemistry, the magnitude of the repulsive electrostatic potential depends on the magnitude of the surface charge obtained at a certain pH and on the concentration of counterions.
The DLVO theory, well known to the colloid chemist, adds the van der Waals attractive potential and the repulsive electrostatic potential to produce a combined interparticle potential that can either be repulsive or attractive, depending on the magnitude of the repulsive potential [see generally, Horn, Roger G., "Surface Forces and Their Action in Ceramic Materials," J. Am. Ceram. Soc. 73(5):1117 (1990)]. One form of a combined interactive potential (high surface charge and low salt content) is shown in FIG. 1b. For this condition, as particles approach one another, they encounter a repulsive energy barrier. The particles repel one another if the energy barrier is greater than their kinetic energy. When the volume fraction of particles is increased sufficiently such that they crowd together, the particles attempt to `sit` at positions that minimize their interaction potential, viz., at a separation distance (usually&gt;20 nm) to form a non-touching, but interactive network schematically shown in FIG. 1b. As the particles are not touching one another, this is called a `dispersed` network.
If the repulsive component of this combined potential is reduced (e.g., by decreasing the surface charge through a change in pH), a condition can be achieved where the repulsive barrier is no longer sufficient to prevent particles from slipping into the deep potential well caused by the van der Waals attractive potential so as to produce the strongly cohesive, touching network described above. According to the DLVO theory, particles should always fall into a deep potential well to form a cohesive flocced network when conditions are much less than optimum, even though the combined repulsive interaction (magnitude of the repulsive barrier and equilibrium separation distance) may be controlled and optimized by, e.g., controlling pH. DLVO theory thus offers no help in understanding whether or how one might control the depth of the attractive well.
It is an object of the present invention to provide techniques for preparing ceramic products from particles which have been as densely packed as possible, using a minimum amount of non-ceramic additives (such as binders and plasticizers).
It is a further object of the present invention to provide manufacturing technology capable of producing high-volume, near-net-shape powder compacts that can be subsequently densified by heat treatment.
It is an additional object of the present invention to provide methods which permit the formation of complex shapes during initial stages of processing that are within a tolerance envelop of the desired shapes after the final stages of processing, such that any shrinkage associated with densification might change the dimensions but not the shape of the final product.
It is yet another object of the present invention to provide fabrication methods for preparation of dense, complex-shaped ceramic products, which methods are simple and inexpensive to use, yet able to handle complex shapes.